1. Field of the Invention
The present invention relates to a bismuth-substituted rare-earth iron garnet single crystal which is used as a Faraday rotator applied to magneto-optic sensors and optical isolators, and more particularly to a method of manufacturing a bismuth-substituted rare-earth iron garnet single crystal film in the form of a flat film formed on a non-magnetic garnet substrate.
2. Description of the Related Art
In recent years, optical fiber communications and optical instrumentation have made remarkable progress. Semiconductor lasers are widely used as a signal source in the optical fiber communications and optical instrumentation. However, semiconductor lasers are disadvantageous in that the oscillation becomes unstable due to so-called reflected light return. In other words, the light is reflected by, for example, the end surface of the optical fiber back to the semiconductor laser. In order to solve this drawback, an optical isolator is usually provided on the light-exiting side of the semiconductor laser. The optical isolator blocks the reflected light return, thereby stabilizing the oscillation of the semiconductor laser.
An optical isolator includes a polarizer, analyzer, Faraday rotator, and permanent magnet. The permanent magnet causes the Faraday rotator to magnetically saturate. The Faraday rotator plays a major role in the optical isolator and is usually formed of a bismuth-substituted rare-earth iron garnet single crystal (referred to as BIG hereinafter) having a thickness in the range from several tens to 550 .mu.m, grown by the liquid phase epitaxial method (referred to as LPE method hereinafter). Proposed single crystals are, for example, (HoTbBi).sub.3 Fe.sub.5 O.sub.12 and (TbBi).sub.3 (FeAlGa).sub.5 O.sub.12.
Conventionally, the BIG is grown by the LPE method as follows: The following materials were introduced in a precious metal crucible: oxides such as ferric oxide and rare-earth oxides, and flux components including lead oxide, boron oxide and bismuth oxide, which are the compositions of a rare-earth iron garnet. The crucible was placed in the middle of the vertical furnace for liquid phase epitaxy and heated to about 1,000.degree. C., thereby melting the materials into a melt for growing a BIG. The melt is then cooled to about 800.degree. C. so as to maintain the rare-earth iron garnet compositions at super saturation.
Then, a non-magnetic garnet substrate is attached to a substrate holder and slowly lowered from the upper part of the LPE furnace until the non-magnetic garnet substrate comes into contact with the surface of the melt. The substrate is then rotated in contact with the melt so that a garnet single crystal is epitaxially grown on the undersurface of the substrate. After the garnet single crystal having a predetermined thickness has grown, the substrate is lifted about several centimeters above the surface of the melt and is spun at a high speed to spin-remove the most of the melt adhering to the substrate. Then, the substrate is taken out from the LPE furnace.
The thus obtained BIG is subjected to the polishing process to separate the substrate from the BIG. During the polishing process, the BIG is polished to a desired thickness. Then, anti-reflection films are formed on both sides of the crystal by vacuum vapor deposition. Finally, using a dicing machine or a scriber, the crystal film is cut into sizes in the range from a 1 mm square (1 mm.times.1 mm) to a 2 mm square (2 mm.times.2 mm), which are the sizes of Faraday rotator for optical isolators.
Semiconductor lasers have a variety of wavelengths. For long-distance optical fiber communications, wavelengths such as 1.31 .mu.m and 1.55 .mu.m (referred to "long wavelength regions") are used since they exhibit low loss in quartz optical fiber. For (HoTbBi).sub.3 Fe.sub.5 O.sub.12, the thickness of a Faraday rotator is about 250 .mu.m at a wavelength of 1.33 .mu.m and about 360 .mu.m at a wavelength of 1.55 .mu.m. The thickness decreases by about 50 .mu.m while the crystal film is subjected to polishing process. Therefore, the BIG should be grown about 50 .mu.m thicker than the final thickness. That is, the crystal film should be grown to a thickness of about 300 .mu.m for a wavelength of 1.33 .mu.m and a thickness of about 410 .mu.m for a wavelength of 1.55 .mu.m.
BIGs have larger coefficients of thermal expansion than non-magnetic garnet substrates to be used and it is therefore difficult to manufacture a BIG on a commercial basis, which BIG has a thickness large than 300 .mu.m and no warp at room temperature.
In other words, the BIG is usually warped at room temperature. In the present invention, the direction of warp of a substrate is "positive" or "+" if the BIG with a substrate thereon is convex upward when it is held in a horizontal plane with the substrate side facing up. Although the lattice constants of the substrate and BIG are almost the same during the growth of crystal, the crystal film is warped at room temperature due to the fact that the BIG shrinks more than the substrate when the BIG with the substrate thereon is cooled down. The magnitude of warp depends on the thickness of the substrate. The larger the thickness of the substrate, the smaller the warp of the substrate. However, the substrate becomes more expensive with increasing thickness and requires a longer time when the BIG with the substrate is polished to remove the substrate. Thus, a thin substrate is desirable. A thin garnet substrate that is currently available is 400 .mu.m for a 2-inch size and 500 .mu.m for a 3-inch size.
When such a thin substrate is used on a commercial basis, it is desirable to maintain the warp of the BIG at room temperature in the range from +0.3 m.sup.-1 to +0.7 m.sup.-1, defined in terms of the reciprocal of a radius of curvature. The substrate is apt to crack during the crystal growth, if the conditions for growing the BIG are selected such that the radius of curvature is not larger than +0.3 m.sup.-1 at room temperature, probably because there occurs a large mismatching in lattice constant between the substrate and the BIG during the crystal growth and therefore a negative component of the warp increases. Conversely, the conditions for growing the BIG may also be selected such that the radius of curvature is larger than +0.7 m.sup.-1 at room temperature in which case, a mismatching in lattice constant between the substrate and the BIG during the crystal growth is small and therefore there is no chance of the substrate cracking. However, when the BIG is cooled down to room temperature, the warp becomes too large due to the difference in coefficient of thermal expansion, increasing the chance of the BIG cracking.
If a 2-inch BIG or a 3-inch BIG has a warp in the range from +0.3 m.sup.-1 to +0.7 m.sup.-1, the BIG cracks during polishing process. Thus, the BIG must be cut into sizes smaller than a 10 mm square before polishing process. Cutting a BIG into smaller sizes creates a problem that a large number of small BIGs must be handled during the polishing process and the subsequent antireflective coating process, and a problem that the yield of final chips is smaller since the BIGs of an intermediate size are further cut into final chip sizes for a Faraday rotator.